High-frequency electric transmission lines or wave guides



June 19, 1951 J. coLLARD HIGH-FREQUENCY ELECTRIC TRANSMISSION LINES 0R WAVEGUIDES Filed July 21, 1945 ffm.

' INVEN TOR. Jam 50u/m12 Patented June 19, 1951 HIGH-FREQUENCY ELECTRIC TRANSMIS- SION LINES R WAVE GUIDES John Collard, HammersmithfLondon, England,

assigner to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application July 21, 1945, Serial No. 606,453

In Great Britain September 14, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires September 14, 1963 6 Claims. 1

vention there is provided a high-frequency transmission line or waveguide wherein at least a part of the surface of the line or guide which serves to confine energy travelling through said line or guide is composed effectively of a plurality of conductors insulated from one another and closely spaced so as to prevent substantial escape of energy, said conductors being of such dimensions and are so arranged with respect to portions of said line or guide or with respect to one another as to provide transmission line sections which, in respect of energy transmitted along said line or guide. appear to be short-circuited at said surface, whereby said conductors simulate in respect 0f said energy a substantially continuous conducting surface.

According to another feature of the invention there is Provided a high-frequency transmission line or waveguide having an opening therein which is covered by a grid which serves to prevent a substantial escape of energy, said grid being insulated from the line or guide and having conducting elements which co-operate with portions of said line or guide to form transmission line sections which, in respect of energy trans- .tted along said line or guide, appear to be Aprt-circuited at said opening whereby said grid causes said opening to appear in respect of said energy to be a substantially continuous conductingsurface.

In certain forms of the invention the transmission line or waveguide has an opening in its surface having extensions projecting from opposite sides of said opening. said conductors extending across said opening and having end sections which co-operate with said expansions to afford said transmission iine sections. Said conductors may be eiectively afforded by a continuous length of conductor extending back and forth vacross said opening and extensions or may be afforded by a conducting coating on an insulating base.

In other forms of the invention the transmission line or waveguide may comprise a pair of tubular conductors having spaced flanged ends, the space between said ends being surrounded effectively by said plurality of conductors which co-operate with said flanged ends to afford said transmission line sections. In this case the said conductors may be effectively afforded by a length of conducting tape which may be helically wound between said flanges or wound back and forth between said flanges, the thin side of the tape being presented towards the longitudinal axis of said transmission line or waveguide.

According to another feature of the invention there is provided a flexible waveguide comprising a flexible insulating member having a plurality of conductors extending through the wall thereof, the length of said conductors being such that adjacent conductors co-operate to form transmission line sections which, in respect of energy transmitted along said guide, appear to be shortcircuited at their inner ends, whereby said conductors simulate a substantially continuous conducting surface.

A section of waveguide or transmission line formed in accordance with the invention may be arranged to pass only those frequencies in respect of which the aforesaid transmission line sections appears to be short-circuited at their inner ends. 25 Thus, a section of waveguide in accordance with the invention may be employed as a filter for eliminating unwanted frequencies includingD. C. Moreover, in apparatus according to the invention the aforesaid conductor or grid will be directly heated by the currents excited by the wave travelling through the apparatus and their temperature will afford an indication of the power in the line or guide. For power measurement purposes the elements are preferably formed of material having a suitable coefcient of variation of resistance with temperature and the elements may be included in a suitable bridge circuit for measuring their resistance to afford an indication of the temperature of the conductors or grid and consequently of the power in the line or guide.

In order that the said invention may be clearly understood and readily carried into effect, it will now be described with reference by way of example to the accompanying drawings, in which:

Figure 1 is a diagrammatic perspective view in section of a waveguide according to the invention,

Figure 2 is a diagrammatic sectional view of the section of waveguide of Figure 1 at the mid-plane perpendicular to the'axis of the guide,

Figure 3 is a detail view showing an alternative form of the plurality of conductors employed in Figures 1 and 2,

Figure 4 is a diagrammatic view in side elevation of a transmission line embodying the inu vention,

Figure' is a side elevational view partly in ing the conductors formed in a different manner.

Figure 1 is a sectional view in the plane VII-VII of Figure 6, and

Figure 8 is a fragmentary perspective view of a flexible waveguide constructed in accordance with the invention.

Referring to Figures 1 and 2 of the drawing, a waveguide is shown of rectangular cross-section having wide sides 2 and 3 shown horizontally and narrow sides 4 and 5 shown vertically. The guide is intended for transmitting waves in the Hw mode, the waves being transmitted within the space enclosed by the inner surface of the guide in well-understood manner. Each narrow side 4 and 5 is provided with a rectangular opening 6 having its longer sides horizontal, the two openings 6 being in registry, as will be seen from Figure 2. Above and below the openings 6 the walls 4 and 5 are formed with extensions 1. The openings 6 are covered eiectively by a plurality oi conductors. The conductors in the example shown in Figures l and 2 are arranged to form grids 8, each grid being formed of a continuous length of insulated wire bent back on itself for a number of times, the lengths of conductors 8a between successive bends extending vertically from the top of the upper extension 1 to the bottom of the lower extension 1. The electrical lengths of the conductors 8a between their upper or lower ends and the upper or lower edge of the opening 6 are made equal to a quarter of a wavelength at the operating frequency of the guide, due allowance being made for the presence of the insulation. Each opening 6 is thus covered with a grid formed of a series of parallel conductors 8a which have end sections extending for a quarter wavelength above and below each opening.

If the waves are propagated through the guide in the H10 mode, the currents flowing in the two narrow sides of the guide will ow at right angles to the axis of the guide. Thus, each end section of the conductors 8a co-operates with the adjacent portion of the wall 4 or 5 and the appropriate extension 1 to provide a two-conductor electrical circuit which is a quarter of a wavelength longr at the operating frequency and is open-circuited at the far end. Thus, as seen from within the waveguide, the conductors 8a appear to be short-circuited to the wall 4 or 5 at the edges of the opening 6, and in respect of highfrequency currents, each grid 8 acts as though it were made up of a grid of conductors having separate elements crossing the opening 1, each element being bonded to the wall of the guide at the edges of the opening 6. Thus, in respect of high frequencies, if the conductors 8a are suiilciently close together so that no radiation takes place through their interstices, each grid 8 replaces the piece of guide cut away to form the opening 6 and the guide thus behaves to high-frequency currentspractically as though no apertures were there. l

When power flows through a section of waveguide the loss in the section of guide causes the temperature of the guide to rise and the increase in the temperature of the guide over the ambient temperature is a measure of the power passing through the guide. It has already been proposed to make use of that fact by wrapping an insulated winding of suitable wire round a section of the luide so that when power `passes through the so as to cause the resistance oi.' the wire to change.

The change of resistance can then be measured by including the wire in a suitable electric bridge circuit in well-known manner so as to afford sn indication of the power passing through the guide. In this case, as the insulated wire is heated by conduction of heat from the guide to the wire through the insulation, the transfer of heat is slow. However, the form of guide shown in Figures 1 and 2 may be employed for power measurement purposes by inserting the grids 9 in bridge circuits without thisdisadvantage arising, as in this case each grid 8 will be traversed by currents excited by the waves and will consequently be heated directly. The measurement can thus be eiected more rapidly than with the known arrangements.

Moreover, as the wire forming each grid is quite long, its D. C. or low-frequency resistance can readily be made quite high, so that i! the wire has a large coeillcient of change of resistance with temperature, the arrangement will have a good sensitivity. At the same time, the grids 8 will have a low resistance to the high-frequency waves travelling in the guide and will not greatly disturb the operation of the guide. The power measuring apparatus can thus be employed to afford a continuous indication when the guide is in use.

A convenient method of constructing the grid 8 is illustrated in Figure 3, which is a detail view of a single grid. In this case the grid is formed on a support of insulating material such as mica of a size such that the support can be applied to one side of the waveguide to extend from the top of the upper extension 1 to the bottom of the lower extension 1 and to cover the opening 6 in the wall of the guide. The insulating support is coated or plated with a illm of conducting material such as platinum having a good coefficient of variation of resistance with temperature, and the coated or plated support is provided with in- `cisions 9 extending from the top and bottom edges alternately as indicated at 9 in Figure 3. the incisions being of such length that when the sheet is assembled the incisions extend from edge to edge of the opening 6 over which the grid is applied'. The conductors 8a of the grid are then constituted by the strips of conducting nlm lying between the overlapping portions of the incisions 9, the grids so formed having a high D. C- resistance and a good coefllcient of variation of resistance with temperature. Preferably, the conducting film on the portions of insulating support lying above and below the upper and lower edges respectively of the opening 6 is made thicker than on the portions of the sheet extending over the opening 6 so that the resistance of the conducting lm in those parts which c0- operate with the extensions 1 on the guide to provide quarter wave line sections is decreased.

'I'he grids 8 of the form shown in Figure 3 are applied to the waveguide with the support outwards, and to insulate the conducting coating from the side oi' the guide the extensions 1 may have a coating of insulating material. 'I'he nature of this material will affect the dimensions oi.' the parts of the grids required to co-operate with the extension 1 on the guide to provide line sections having the required electrical length. With a grid oi.' the kind shown in Figure 3, no diiliculties will be occasioned due to radiation through the gaps between the conductors, as

.these gaps can be made sufllciently narrow to bient temperature, but not to be affected by waves passing through the guide or by temperature changes confined to the guide.

In the case `of a rectangular guide used for the transmission of waves in the Hoi mode, the openings such vas 6 should be provided 4in the wide` sides l and and not in the narrow sides, these openings being covered with grids such as 8 c`ooperating with extensions of the wide sides similar to the extensions 1 of Figures 1 and 2.

Figure 4 illustrates the invention in application to a section of transmission line ofthe coaxial conductor type having cylindrical inner and outer conductors I0 and II respectively. The outer conductor Il is provided with two diametrically opposed openings I6 corresponding to the openings 8 of Figures 1 and'2 and provided with flanges I1 on the edges at right angles to the axis of the line. Between the flanges I1 are mounted insulated grids I8 corresponding to the grids 8 of Figures 1 and 2, each conductor I8a of the grids comprising a portion extending across the associated opening I6 parallel to theaxis of the system and having end sections extending radially across each ofthe flanges I1 associated with each opening. 'I'he flanges I1 and the cooperating portions. of the elements I 8a extend radially over a quarter of a wavelength at the operating frequency .of the guide.

In the case of a co-axial line, currents travel f in-the outer conductor II in a direction parallel to the axis of the line instead of at right angles to the axis as theydo in the narrow sides 4 and 5 of the rectangular'guide' of Figures 1 and 2, and hence in the arrangement shown in Figure 4 the grids I8 have their'conductors I8a extending across the apertures I6 in a direction parallel to the axis of the line. In the arrangement of Figure 4 the grids I8 act in a manner similar to that in which grids 8 of Figures 1 and 2 act and the grids I8 may be regarded as effectively replacing the pieces of conductor II removed to form the opening I6. However, as the grids are insulated from the conductor I I and are traversed by currents excited by the wave flowing through the guide they are available for power measurement like the grids 8 of Figures 1 and 2.

In a modification of the arrangement of Figure 4 the outer conductor II `may be replaced by a pair of hollow conductors having anged ends presented to each other, and the two grids I8 may be replaced by a grid forming a cylindrical cagelike structure and affording, in effect, a continuation of the surfaces of the hollow conductors.

Figure 5 shows the application of the invention to awaveguide formed of hollow cylindrical conductors 2| and 22. In the case of guides of this form the current flow is generally too complex, except for the Ho mode, to permit the use of a simple wire grid. 'I'he conductors 2I and 22 are therefore provided with radial end flanges 21 and these flanges co-operate with a plurality of effective conductors. These conductors are effectively provided by a length of conducting tape 28 wound to form a helix, as shown, having an inner-diameter the same as that of the outer diameter of the hollow conductors 2lv and 22, the flanges 21 and the conductor 28 having a widthsuch as to afford an electrical length'of-'a quarter of the wavelength at the operating frequency.v

The. tape is wound with its thin side directed towards the longitudinal axis of the guide. with the arrangement of Figure 5 itwill be appreciated that at any point on the inner edges o1' the flanges*- 21 of conductors 2| and 22 the appropriate flanges 21 and the surface of the adjacent turn of the conductor 28 constitute a two-conductor circuit open-circuited at the end remote from the point and extending radially outwards-from the point. .Thus, in respect of waves trans-- mitted through the guide. any point on the inner edge of flange 21 may be considered as being short-circuited to the nearest point on the inner edgeof the adjacent turn of conductor 28 so that the flange 21 and the turn of conductor 28 ad- 4jacent the flange may be considered as directly connected in respect of wave transmission.' Similarly, any point on the inner edge of each turn oi?v the conductor 28 may be regarded as directly connected to the nearest points on the inner edgeof the neighboring turns in respect of wave transmission, and as a result thev two conductors 2I and 22 and the conductor 28 appears as a continuous tubular conductor in respect of wave transmission. If the turns of conductor 28 are sufficiently closely spaced there will be no appreciable radiation through the spaces between the turns.

To insulate the conductor 28 insulating material 29may be formed about the conductor or one or each side of the conductor may have an insulating strip or layer applied to it. It will be appreciated thatl the width of the conductor 28 and the flanges 21 will be such as to give an elictrical quarter Wavelength with the particular insulating material 29 which is employed. In the arrangement of 'Figure 5 a wave travelling through the guide constituted by the conductors 2 I, 28.and 22 will excite currents in the conductor 28 which will become directly heated. Thus, the conductor 28 may be included in a resistance bridge and employed for power measurement as in the case of the grids 8 and I8 of Figures 1 to 4. Moreover, it is to be observed that in the arrangement of Figure 5, while the conductor 28 provides a connection between yconductors 2| and 22 in respect of wave transmission, no such connection y is provided in respect ofD.-C. Thus the invention provides an arrangement forming a waveguide having sections insulated one from the other in respect of the transmission of D.C. Moreover, the turns of the conductor 28 will only appear short-circuited from their inner edges in respect of a particular frequency and certain harmonics of that frequency so that the conductor will not operate to transfer waves of all fr..- quencies, but will act as a lter.

In the embodiment of the invention shown in Figures 6 and 7 the conductor 28, instead of being wound on a helix, is wound in lengths which are arranged to extend back and forth between flanges 21 of conductors 2| and 22 parallel to the axes of the guide, the conductor 28 beingbent back on itself at the end of each length. In this flanges 21 in respect of wave transmissionsv along the guide.

Figure 8 is a diagrammatical representationV conductive surface.

a 7 of a flexible waveguide made in accordance with the invention. This guide consists of a tube Il of flexible insulating material, such as rubber, the inside surfaces of the tube constituting the operative surfaces of the guide and the thickness of the wall of the tube being made such as to aiford an electrical length of a quarter wavelength at the operating frequency of the guide between the inner and outer face of each wall. The tube 3| may be rectangular as shown or of other cross section. A large number of conductors, such as wires I2, are inserted normally in the walls of the guide 3l, each wire having a length equal to the thickness of the walls of the guide so that the wire extends from one surface to the other of the wall in which it is inserted. With this arrangement each pair of adjacent wires, together with the rubber or other dielectric surrounding it, as seen from the inner surface of the guide, presents a section of transmission line extending outwardly from said surface and having an electrical length of a quarter wavelength and open-circuited at its remote end so that the inner ends of the wires appear to be directly connected together in respect of wave transmission along the guide and simulate a continuous electrically- Thus, the guide will operate substantially as though it were constructed entirely of conducting material.

The wires 32 may be inserted in the guide 3| of Figure 8 by means of a hollow steel needle which is used to pierce the wall and through which the wire is fed into the rubber so as to be gripped therein when the needle is removed.

It will be appreciated that while in the foregoing description the extensions 1 and 8a of Figures l, 2 and 3, and the flanges Il and elements Ila of Figure 4, the flanges 27 and the conductor 28 of Figures 5 and 7, and the wires 32 of Figure 8 have been arranged to form quarter wavelength transmission lines, they might be formed to provide lines having lengths which are odd multiples of a quarter of a wavelength. Also the conductors referred to might be arranged to provide lines which are short-circuited at their outer ends and have lengths equal to a Whole number of half wavelengths.

Moreover, while in the foregoing examples the invention has been shown as applied to hollow transmission lines or guides, it will be understood that the interior of these members might be filled with dielectric material and in the case of the arrangement of Figure 8, the member 3| might be replaced by a rigid insulating material.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. A high-frequency transmission waveguide, a portion of the energy confining surface thereof .8 having an aperture therein, extensions oi' said surface arranged in the proximity of said aper- .ture and a continuous conductor .insulated from and arranged upon said extensions and extending back and forth across said aperture, coincident portions of said extensions and said conductor being a quarter wavelength long at the frequency of the energy transmitted whereby the portion of said energy conilning surface oonstituted by said conductor appears to be shortcircuited at said surface with respect to the energy transmitted along said waveguide and the temperature variations of said conductor when energy is transmitted through said guide serves an indication of the power transmitted therethrough.

2. The combination claimed in claim l, said continuous conductor comprising a single wire.

3. The combination claimed in claim 1, said continuous conductor comprising a. ilhn of conducting material having incisions.

4. The combination claimed in claim 1, said waveguide being a rectangular hollow pipe waveguide having a pair of parallel broad walls and a second pair of parallel walls narrower than and normal to said broad walls, the inner surface of said walls being said energy confining surface, said aperture being in said narrow walls.

5. The combination claimed in claim l, said waveguide having a circularly cylindrical pipe the inner cylindrical surface of which is said energy confining surface.

6. The arrangement claimed in claim 1, said continuous conductor having a known coefilcient of change of resistance with respect to temperature, whereby a resistance measurement of said conductor is a measure of said power.

JOHN COLLARD.

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